Data Sheet

1012
Atmel | SMART SAM D21 [DATASHEET]
Atmel-42181G–SAM-D21_Datasheet–09/2015
38.6 Clocks and Crystal Oscillators
The SAM D21 can be run from internal or external clock sources, or a mix of internal and external sources. An example
of usage will be to use the internal 8MHz oscillator as source for the system clock, and an external 32.768kHz watch
crystal as clock source for the Real-Time counter (RTC).
38.6.1 External Clock Source
Figure 38-5. External Clock Source Example Schematic
38.6.2 Crystal Oscillator
Figure 38-6. Crystal Oscillator Example Schematic
The crystal should be located as close to the device as possible. Long signal lines may cause too high load to operate
the crystal, and cause crosstalk to other parts of the system.
Table 38-5. Crystal Oscillator Checklist
Notes: 1. These values are given only as typical example.
2. Decoupling capacitor should be placed close to the device for each supply pin pair in the signal group.
Table 38-4. External Clock Source Connections
Signal Name Recommended Pin Connection Description
XIN XIN is used as input for an external clock signal Input for inverting oscillator pin
XOUT/GPIO Can be left unconnected or used as normal GPIO
XOUT/GPIO
XIN
NC/GPIO
External
Clock
Signal Name Recommended Pin Connection Description
XIN Load capacitor 15pF
(1)(2)
External crystal between 0.4 to 30MHz
XOUT Load capacitor 15pF
(1)(2)
XOUT
XIN
15pF
15pF